function Bullet(src,w,h,x,y){
    Sprite.call(this,src,w,h,x,y)
}
Bullet.prototype = new Sprite;

Bullet.prototype.move = function(speed){
  this.imgNode.style.top = this.imgNode.offsetTop - speed + 'px'
  this.y = this.imgNode.offsetTop;
}
